Photographing with a still camera in a dark environment often causes a camera shake because of a long exposure time. In order to suppress a camera shake, an image capturing apparatus has conventionally been proposed which can obtain a focused image without decreasing the image quality.
For example, first, an image capturing apparatus disclosed in Japanese Patent Laid-Open No. 9-261526 sequentially captures low-luminance images in an exposure time during which no camera shake substantially occurs. Then, the image capturing apparatus corrects a positional error of each captured image, performs a composition process based on superposition, and finally generates one image of proper luminance. The image capturing apparatus can provide a focused image even if a camera shake occurs with a lens of a long focal length or in a long exposure time.
The image capturing apparatus disclosed in Japanese Patent Laid-Open No. 9-261526 will be briefly described.
First, the image capturing apparatus calculates, on the basis of a photographing focal length f in the optical system of the image capturing apparatus, the time of limit seconds t(f) serving as the upper limit of an accumulation time ta during which no camera shake occurs. Then, if ta ≧t(f) for the accumulation time ta obtained on the basis of the photometry result, the image capturing apparatus sets again the accumulation time ta to an arbitrary time which is equal to or shorter than the time of limit seconds t(f). The image capturing apparatus sets a sequential shooting count k in accordance with the reset value of the accumulation time ta, and executes sequential shooting by the sequential shooting count k. The image capturing apparatus corrects deviations between sequentially shot images, and composites them into one image of correct exposure. In this manner, the accumulation time can be controlled to suppress degradation of the image quality of a photographed image that is caused by a camera shake in photographing.
Another image capturing apparatus has conventionally been proposed which suppresses flicker generated when an image of an object is captured under illumination light which flickers at an interval different from the image capturing interval (an example of illumination light is a fluorescent lamp).
For example, when the flickering interval of illumination light which illuminates an object is different from the image capturing interval, an image capturing apparatus disclosed in Japanese Patent Laid-Open No. 6-78223 makes the timing when the illuminance of illumination light reaches a maximum value, coincide with the center of the charge accumulation period of a solid-state image capturing element. This suppresses flicker of an image caused by flickering illumination light.
The image capturing apparatus disclosed in Japanese Patent Laid-Open No. 6-78223 will be briefly described.
FIG. 9 is a graph showing an electronic shutter control method in the image capturing apparatus disclosed in Japanese Patent Laid-Open No. 6-78223. In FIG. 9, reference numeral 901 denotes an illuminance level representing a change in the brightness of illumination light; and 902, a ground level. Reference numeral 903 denotes a phase of the minimum value of the illumination light quantity; and 904, a phase of the maximum value. Reference numeral 905 denotes a charge accumulation period of a solid-state image capturing element. The image capturing apparatus disclosed in Japanese Patent Laid-Open No. 6-78223 sequentially captures images. As operation of the image capturing apparatus, first, the image capturing apparatus detects the phases 904 of maximum values of the illumination light quantity in preceding fields, and stores a phase at which operation of an electronic shutter starts in the past three fields. Then, the image capturing apparatus predicts and controls the operation start time of the electronic shutter so that the center of the charge accumulation period of the solid-state image capturing element coincides with the phase 904 of the maximum value of the illumination light quantity. The electronic shutter is controlled to suppress flicker of an image.
Further, for example, an image capturing apparatus disclosed in Japanese Patent Laid-Open No. 1-34070 detects the flickering frequency of illumination light, adjusts the exposure time of an image capturing element to the flickering interval of illumination light, and thereby suppresses flicker of an image that is caused by an electronic shutter.
The image capturing apparatus disclosed in Japanese Patent Laid-Open No. 1-34070 will be briefly described.
When the image capturing method is NTSC at a field frequency of 60 Hz and the frequency of external light is 50 Hz (i.e., the flickering frequency of the light quantity is 100 Hz), the exposure time is set to 1/100 [sec] . When the image capturing method is PAL and the frequency of external light is 60 Hz (i.e., the flickering frequency of the light quantity is 120 Hz), the exposure time is set to 1/120 [sec]. The exposure time is controlled to suppress flicker of an image.
The image capturing apparatus disclosed in Japanese Patent Laid-Open No. 9-261526 may pose a problem if a plurality of images used for the composition process are discretely captured. For example, when the exposure time is shorter than the flickering interval under illumination light containing flicker, the minimum value of the illuminance of illumination light may fall within the exposure time but no maximum value may fall within it. For this reason, the luminance of an image generated by the composition process may become insufficient. To the contrary, when the exposure time is longer than the flickering interval under illumination light containing flicker, the number of minimum values of the illuminance of illumination light during the exposure time may become larger than that of maximum values. As a result, the luminance of an image generated by the composition process may become insufficient. Owing to these problems, when a picture is taken under a light source containing flicker by using the image capturing apparatus disclosed in Japanese Patent Laid-Open No. 9-261526, no image of correct exposure can be finally obtained depending on the setting of the photographing timing or the setting of the exposure time.
In the image capturing apparatus disclosed in Japanese Patent Laid-Open No. 6-78223, the luminance of an obtained image may decrease when the exposure time is longer than the flicker interval under illumination light containing flicker. For example, under a photographing condition that an exposure time T has a relation with a flicker interval f:(2n−1)f<T<2nf  (1)
(n is a natural number) if the timing when the illuminance of illumination light reaches a maximum value is made to coincide with the center of the exposure period, the number of minimum values of the illuminance of illumination light during the exposure time may become larger than that of maximum values. Hence, the luminance of an obtained image may become lower than that of an image photographed at the average illuminance of illumination light containing flicker. In addition, under a photographing condition that the exposure time T has a relation with the flicker interval f:2nf<T<(2n+1)f  (2)
(n is a natural number)
if the timing when the illuminance of illumination light reaches a maximum value is made to coincide with the center of the exposure period, the number of minimum values of the illuminance of illumination light during the exposure time may become smaller than that of maximum values. The luminance of an obtained image may become higher than that of an image photographed at the average illuminance of illumination light containing flicker.
The image capturing apparatus disclosed in Japanese Patent Laid-Open No. 1-34070 may pose a problem depending on the photographing condition when the exposure time is adjusted to the flicker interval under illumination light containing flicker. For example, if the flicker interval is longer than an exposure time during which no camera shake is predicted to occur, a camera shake may occur because an actual exposure time is set in accordance with the flicker interval.